I have a jfet based preamp that really makes a racket on turn on. It runs off 24vdc. Would putting a small value .22uf/630v capacitor across the power on switch be a way to control this noise?

Jan Dupont

16th February 2010 01:44 PM

It depends on what makes the turn on noise. The power switch or your jfet preamp. But try the cap. It may solve the problem ;)

Michael Chua

16th February 2010 02:16 PM

0.22uF is quite large. Use a 10nF /240Vac Class X or X2 Suppressor Cap across the power switch contacts. Silent ON/OFF.

Regards
Mike

DreadPirate

16th February 2010 05:05 PM

I'll give it a try. Can same approach be used across a 110v switch?

Jan Dupont

16th February 2010 05:11 PM

Quote:

Can same approach be used across a 110v switch?

Yes. Only difference is the Voltage rating of the cap.

Michael Chua

16th February 2010 05:15 PM

No problem with 110V switch. What is important is to use CLASS X or X2 caps. These are specially manufactured (approved) for Mains use.

Speedskater

17th February 2010 12:17 AM

Quote:

Originally Posted by Michael Chua
(Post 2087549)

No problem with 110V switch. What is important is to use CLASS X or X2 caps. These are specially manufactured (approved) for Mains use.

Because (and this is especially true with bigger power amplifiers) if you switch off at the wrong time in the power line cycle (when the transformer field is at maximum) The transformer field collapses, but the current has no-where to go (the switch is now open) so the primary voltage heads towards infinity! Only the circuit leakages limit this voltage, but it's all across the capacitor.

Now in real life things aren't this bad, but it does stress the capacitor.

Speedskater

17th February 2010 12:26 AM

Quote:

Originally Posted by Michael Chua
(Post 2087549)

No problem with 110V switch. What is important is to use CLASS X or X2 caps. These are specially manufactured (approved) for Mains use.

And the other reason is because the capacitor is always across line voltage when the unit is switched off.

Mooly

18th February 2010 11:15 AM

Quote:

Originally Posted by DreadPirate
(Post 2087368)

I have a jfet based preamp that really makes a racket on turn on. It runs off 24vdc. Would putting a small value .22uf/630v capacitor across the power on switch be a way to control this noise?

Have you tried a slow start (power transistor/resistor and cap) as you say it runs off 24 Vdc, to get the rail to rise slowly.

Or connect a normally closed relay across the output shorting to ground, and again arrange for small delay before it opens.

Is the circuit "oscillating" as the DC conditions stabilise ?

DreadPirate

18th February 2010 11:52 AM

I'm going to try the capacitor trick, if not, where is a power trans/resistor/cap circuit I can try? Would it power off the 24vdc, I don't want to provide another power source just for it. Using the 555 and relay is a bit more complicated than can be justified for this project.